Applied research has been vigorously made on an organic EL device as, a high-speed responsiveness and high-efficiency light-emitting device (Macromol. Symp. 125, 1-48 (1997)). FIGS. 1A and 1B each show the basic structure of the device. As shown in FIGS. 1A and 1B, the organic EL device is generally composed of a plurality of organic layers interposed between a transparent electrode 5 on a transparent substrate 6 and a metal electrode 1.
In FIG. 1A, the organic layers are composed of an electron-transporting layer 2, a light-emitting layer 3, and a hole-transporting layer 4.
For example, ITO having a large work function is used for the transparent electrode 5 to provide good property of injecting a hole from the transparent electrode 5 to the hole-transporting layer 4. A metal material having a small work function such as aluminum, magnesium, or an alloy thereof is used for the metal electrode 1 to provide good property of injecting electrons to the organic layers. Those electrodes each have a thickness in the range of 50 to 200 nm.
For example, an aluminum-quinolinol complex (typified by Alq3 shown below) having electron-transporting and light-emitting properties is used for the light-emitting layer 3. In addition, a material having electron-donating property such as a biphenyl diamine derivative (typified by α-NPD shown below) is used for the hole-transporting layer 4. An oxadiazole derivative or the like can be used for the electron-transporting layer 2.
Fluorescence upon transition of a singlet exciton of a molecule as a light-emitting center to a ground state has been heretofore taken as light emission generally used in an organic EL device. Meanwhile, a device utilizing not fluorescent emission via a singlet exciton but phosphorescence via a triplet exciton has been under investigation (“Improved energy transfer in electrophosphorescent device” (D. F. O'Brien et al., Applied Physics Letters Vol 74, No 3, p 422 (1999)) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” (M. A. Baldo et al., Applied Physics Letters Vol 75, No 1, p 4 (1999))). In each of those documents, a four-layer structure composed of organic layers shown in FIG. 1B is mainly used. The four-layer structure is composed of the hole-transporting layer 4, the light-emitting layer 3, an exciton diffusion blocking layer 7, and the electron-transporting layer 2 in this order from the side of an anode. The materials used are the following carrier-transporting materials and phosphorescent materials. Abbreviations of the respective materials are as follows.
Alq3: Aluminum-quinolinol complex
α-NPD: N4,N4′-Di-naphthalen-1-yl-N4,N4′-diphenyl-biphenyl-4,4′-diamine
CBP: 4,4′-N,N′-Dicarbazole-biphenyl
BCP: 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline
PtOEP: platinum-octaethylporphyrin complex
Ir(ppy)3: Iridium-phenylpyridine complex

High efficiency is obtained in each of “Improved energy transfer in electrophosphorescent device” (D. F. O'Brien et al, Applied Physics Letters Vol 74, No 3, p 422 (1999)) and “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” (M. A. Baldo et al, Applied Physics Letters Vol 75, No 1, p 4 (1999)) when α-NPD was used for the hole-transporting layer 4, Alq3 for the electron-transporting layer 2, BCP for the exciton diffusion blocking layer 7, and CBP as a host material for the light-emitting layer 3 which was mixed with PtOEP or Ir(ppy)3 as a phosphorescent material at a concentration of about 6%.
A phosphorescent material has been attracting considerable attention because it is expected to provide high emission efficiency on principle. The reason for this is that excitons generated by carrier recombination are composed of singlet excitons and triplet excitons, and the ratio between the number of singlet excitons and the number of triplet excitons is 1:3. An organic EL device utilizing a singlet has taken fluorescence upon transition from a singlet exciton to a ground state as light emission. However, on principle, the luminescence yield of the device was 25% of the number of generated excitons, and the value was an upper limit on principle. When phosphorescence from an exciton generated from a triplet is used, an yield at least 3 times as high as that of the above yield is expected on principle. Furthermore, when transfer due to intersystem crossing from a singlet at a higher energy level to a triplet is taken into consideration, a light emission yield 4 times as high as the above yield, that is, a luminescence yield of 100% is expected.
In addition, the development of a host material using a phosphorescent compound as a dopant has been actively made (Japanese Patent Application Laid-Open No. 2003-55275). The development of a phosphorescent compound has also been actively made. The development of an iridium complex containing fluorine as a halogen atom (International Publication No. WO 02/02714) has been made. A device which is formed by introducing a fluorine atom into it to provide a dopant concentration of 20% or more has been proposed.
However, an organic EL device utilizing phosphorescence, the organic EL device using, as a light-emitting material, a phosphorescent compound containing a halogen atom in its molecular structure, involves a problem in that the organic EL device has an extremely short device lifetime, although the reason by which such problem occurs is unclear.